DESCRIPTION

A MooseX::Types library for creating parameterizable types. A parameterizable type constraint for all intents and uses is a subclass of a parent type, but adds additional type parameters which are available to constraint callbacks (such as inside the 'where' clause of a type constraint definition) or in the coercions you define for a given type constraint.

If you have Moose experience, you probably are familiar with the builtin parameterizable type constraints 'ArrayRef' and 'HashRef'. This type constraint lets you generate your own versions of parameterized constraints that work similarly. See Moose::Util::TypeConstraints for more.

Using this type constraint, you can generate new type constraints that have additional runtime advice, such as being able to specify maximum and minimum values for an Int (integer) type constraint:

This is useful since it lets you generate common patterns of type constraints rather than build a custom type constraint for all similar cases.

The type parameter must be valid against the 'constrainting' type constraint used in the Parameterizable condition. If you pass an invalid value this throws a hard Moose exception. You'll need to capture it in an eval or related exception catching system (see TryCatch or Try::Tiny.)

For example the following would throw a hard error (and not just return false)

RangedInt([{min=>99, max=>10}])->check(10); ## Not OK, not a valid Range!

In the above case the 'min' value is larger than the 'max', which violates the Range constraint. We throw a hard error here since I think incorrect type parameters are most likely to be the result of a typo or other true error conditions.

If you can't accept a hard exception here, you can either trap it as advised above or you need to test the constraining values first, as in:

Please note that for ArrayRef or HashRef parameterizable type constraints, as in the example above, as a convenience we automatically ref the incoming type parameters, so that the above could also be written as:

This is the preferred syntax, as it improve readability and adds to the conciseness of your type constraint declarations.

Also note that if you 'chain' parameterization results with a method call like:

TypeConstraint([$ob])->method;

You need to have the "(...)" around the ArrayRef in the Type Constraint parameters. You can skip the wrapping parenthesis in the most common cases, such as when you use the type constraint in the options section of a Moose attribute declaration, or when defining type libraries.

Subtyping a Parameterizable type constraints

When subclassing a parameterizable type you must be careful to match either the required type parameter type constraint, or if re-parameterizing, the new type constraints are a subtype of the parent. For example:

Of course the above is somewhat counter-intuitive to the reader, since we have defined our 'RangedInt' in such as way as to let you declare negative ranges. For the moment each type constraint rule is apply without knowledge of any other rule, nor can a rule 'inform' existing rules. This is a limitation of the current system. However, you could instead do the following:

Notice how re-parameterizing the parameterizable type 'RangedInt' works slightly differently from re-parameterizing 'PositiveRange' Although it initially takes two type constraint values to declare a parameterizable type, should you wish to later re-parameterize it, you only use a subtype of the extra type parameter (the parameterizable type constraints) since the first type constraint sets the parent type for the parameterizable type.

In other words, given the example above, a type constraint of 'RangedInt' would have a parent of 'Int', not 'Parameterizable' and for all intends and uses you could stick it wherever you'd need an Int. You can't change the parent, even to make it a subclass of Int.

Coercions

A type coercion is a rule that allows you to transform one type from one or more other types. Please see Moose::Cookbook::Basics::Recipe5 for an example of type coercions if you are not familiar with the subject.

MooseX::Types::Parameterizable supports type coercions in all the ways you would expect. In addition, it also supports a limited form of type coercion inheritance. Generally speaking, type constraints don't inherit coercions since this would rapidly become confusing. However, since your parameterizable type is intended to become parameterized in order to be useful, we support inheriting from a 'base' parameterizable type constraint to its 'child' parameterized sub types.

For the purposes of this discussion, a parameterizable type is a subtype created when you say, "as Parameterizable[..." in your sub type declaration. For example:

This is the "SYNOPSIS" example, which creates a new parameterizable subtype of Str which takes a single type parameter which must be an Int. This Int is used to constrain the allowed length of the Str value.

Now, this new sub type, "Varchar", is parameterizable since it can take a type parameter. We can apply some coercions to it:

This parameterizable subtype, "Varchar" itself is something you'd never use directly to constraint a value. In other words you'd never do something like:

has name => (isa=>Varchar, ...); ## Why not just use a Str?

You are going to do this:

has name => (isa=>Varchar[40], ...)

Which is actually useful. However, "Varchar[40]" is a parameterized type, it is a subtype of the parameterizable "Varchar" and it inherits coercions from its parent. This may be a bit surprising to Moose developers, but I believe this is the actual desired behavior.

You can of course add new coercions to a subtype of a parameterizable type:

In which case this new parameterizable type would NOT inherit coercions from it's parent parameterizable type (Varchar). This is done in keeping with how generally speaking Moose type constraints avoid complicated coercion inheritance schemes, however I am open to discussion if there are valid use cases.

NOTE: One thing you can't do is add a coercion to an already parameterized type. Currently the following would throw a hard error:

subtype 40CharStr,
as Varchar[40];
coerce 40CharStr, ... # BANG!

This limitation is enforced since generally we expect coercions on the parent. However if good use cases arise we may lift this in the future.

In general we are trying to take a conservative approach that keeps in line with how most Moose authors expect type constraints to work.

Recursion

TBD - Needs a use case... Anyone?

TYPE CONSTRAINTS

Create a subtype of ParentTypeConstraint with a dependency on a value that can pass the ConstrainingValueTypeConstraint. If ConstrainingValueTypeConstraint is empty we default to the 'Any' type constraint (see Moose::Util::TypeConstraints). This is useful if you are creating some base Parameterizable type constraints that you intend to sub class.